International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 2. Spontaneous mutations in mammalian cells.

Spontaneous or background mutation in mammals plays an important role in both medical and evolutionary contexts. However, establishing mutation frequencies or rates has not always been easy. When the field of mammalian mutagenesis was in its infancy, the word "variant" rather than "mutant" was often used because the genetic nature of the observed phenotypic alterations could not be adequately proven. Nowadays numerous target genes have been identified in which mutant frequencies can be measured, and occasionally even rates can be estimated. Indeed, the genetic basis for 'variants' now often comes from direct DNA sequencing. This review describes the most often used and best understood genetic markers for mutation research and examines their usefulness. In addition, mutational specificity is compared for several loci and the use of DNA-sequence data in determining the origins of spontaneous mutation is also discussed. An important observation is that spontaneous mutation frequencies of similarly sized genes can vary by more than an order of magnitude. Chromosomal location, the nature of the gene product and mutational specificity may offer a partial explanation.